1. Field of the Invention
The present invention relates to a thin film magnetic head and more particularly to a thin film magnetic head which employs high moment NiFe alloys for the magnetic poles in an inductive write element.
2. Description of the Related Art
Two major types of thin film magnetic heads are the inductive head and the inductive/magnetoresistive (MR) head. Both of these heads can write and read signals with respect to a magnetic medium. The inductive head includes first and second poles which have first and second pole tips respectively. The pole tips are separated by a gap at an air bearing surface (ABS) or head surface. A coil is positioned between the first and second poles. The MR head has an inductive write head portion and an MR read head portion. The read head portion includes an MR stripe which is sandwiched between a pair of gap layers, the gap layers in turn being sandwiched between first and second shield layers. In the MR head the inductive head portion performs write functions and the MR read head portion performs read functions. Either type of magnetic head is mounted on or embedded in a slider which is supported in a transducing relationship with respect to a magnetic medium. The magnetic medium can be either a magnetic disk or a magnetic tape.
Considerable research has been undertaken to increase the recording density of magnetic heads. The length (i.e., the thickness) of the gap between the first and second pole tips has been significantly decreased so that more bits per inch can be written. Further, the coercivity of the magnetic medium has been increased to allow the medium to retain a higher bit density. A consequence of a higher bit density is a higher data rate for information as it is made or written on the medium.
These improvements require the material of the magnetic poles to conduct relatively high flux densities, especially those portions of the poles, i.e., the pole tips, which are adjacent to the gap. However, materials have a saturation level beyond which they will conduct no more flux. Accordingly, there is a need for a pole tip structure which has a high saturation moment (4.pi.M.sub.s), hereinafter referred to as "the moment" of the material.
The first and second pole pieces, including the pole tips, are typically constructed of Permalloy (Ni.sub.81 Fe.sub.19). Permalloy is a desirable material for pole-construction, having good soft magnetic properties and being easy to shape by normal patterning and deposition techniques. Further, Permalloy has good corrosion resistance for head reliability. Permalloy has a moment which is approximately ten kilogauss (10 kG). It would be desirable if this moment could be increased so that the pole tips could carry a larger flux density without saturation. It is also useful if the material's magnetic and electrical properties change so that eddy currents do not interfere with high frequency operation. Eddy current damping is improved by increasing the resistivity of the material as well as decreasing the permeability.
Cobalt based magnetic alloys have an increased moment with respect to Permalloy. However, these materials have significantly worse corrosion resistance. Another family of materials are sputtered FeNX, where X is from the group of Ta, Al, and Rh. This is not as desirable as frame plating the pole pieces since ion-milling is required after the sputtering to shape the trackwidth of the pole tips. This process is very difficult to implement. Furthermore, sputtered materials exhibit a high stress which can cause distortion in recorded signals.
U.S. Pat. No. 4,589,042, assigned to the instant assignee, discloses an inductive read/write head wherein the pole tip regions of the magnetic poles are fabricated of a high moment material while the remainder of the pole structure is of Permalloy. The pole tip regions are made of high moment Ni.sub.45 Fe.sub.55 and extend to a position short of the first coil winding beyond the point of initial saturation of the poles with the balance of the poles being of high permeability material such as Permalloy. However, as discussed below with respect to a MIG head, the Ni.sub.45 Fe.sub.55 has a high positive magnetostriction which adversely impacts the read signal.
Another method of increasing the flux density at which the pole-tips saturate is to employ a metal-in-gap (MIG) configuration of the pole-tips. The prior art teaches constructing one or more of the pole tips with a bilayer configuration, one of the layers having a high moment and other layer a lower moment. The higher moment material is placed adjacent the gap where it is most needed. However, a high moment material may exhibit poor performance if the magnetostriction of the material is not controlled. Magnetostriction significantly affects the domain structure of the material causing the domains to be switched in intervals which leads to poor performance. This is manifested in the inductive magnetic head when it is also employed for reading. In a merged MR head, where the second shield of a MR read head serves also as a first pole of an inductive write head portion, the reading performance is poor if the magnetostriction of the first pole/shield is not controlled.
The desired magnetostriction for pole tip material is zero or slightly negative. It would be desirable if the pole tips could be constructed with a MIG configuration having a high moment and near zero magnetostriction or in a pole design that improves write performance without impacting readback.